Recently, we discovered an inherited immunodeficiency disease in certain hybrid families of chickens referred to as UCD 140. The chief characteristics of the disease are: 1. multifactorial inheritance; 2. acquired (late onset) selective IgG deficiency; 3. increased IgM serum levels producing a dysgammaglobulinemia; 4. increased serum IgG in some birds after several months of age; and 5. manifestations of autoimmune pathology late in the disease. We propose: a. to determine whether the disease is caused by a major gene; b. to correlate the age-dependent relationship of Ig levels, suppressor activity of peripheral blood mononuclear cells, surface and cytoplasmic Ig markers of PBL, development of immune responses, function of T and B cells, and onset of autoimmune manifestations. In addition, the effect of germ-free environment, ability of induced tolerance, and the effect of bursectomy, thymectomy, and splenectomy will be studied. The automimmune diseases will be characterized in detail.